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Lecture 21 - Preprocessor and Header File
- 1. The Preprocessor and Header File Md. Imran Hossain Showrov ([email protected]) 21 1
- 2. Outline What is Preprocessor? Preprocessor Directives Preprocessor Directives Categories Macro Expansion File Inclusion Conditional compilation What is a Header File? Why Header Files are used?
- 3. What is Preprocessor? The preprocessor is a program that processes the source code before it passes through the compiler. Produce a source code file with the preprocessing commands properly sorted out.
- 4. Preprocessor Directives Preprocessor commands are known as directives. Preprocessor provides certain features. These features are also known as preprocessor directives. Preprocessor directives start with # sign. #include <stdio.h>
- 5. Preprocessor Directives (cont..) Preprocessor directives can be placed any where in the source program. Note: Place it at start of the program. Each preprocessor directive must be on it’s own line.
- 7. Preprocessor Directives Categories Preprocessor directives can be divided into three categories: 1. Macro Expansion 2. File Inclusion 3. Conditional compilation (also known as compiler control directives)
- 8. 1. Macro Expansion #define directive is known as macro expansion. Definition: General Form: #define macro_template macro_expansion #define macro_name char_sequence
- 9. 1. Macro Expansion (cont..) Preprocessor search for macro definition. After finding #define directive it search entire program for macro_template. Replace each macro_template with macro_expansion. Best Practice: Use capital letters for macro template. Do not use semicolon ‘ ; ’
- 10. Why Macro Expansion is used? To write efficient programs. To increase readiabiality of programs. Variable vs macro_template Compiler can generate faster and compact code for constant than it can for variables. When you are dealing with a constant, why use variable. A variable may change in the program.
- 11. 2. File Inclusion causes one file to be included in another. <filename> : search the directory on current directory only. “filename” : search the directory on current directory and specified directories as specified in the include search path.
- 12. Why File Inclusion is used? Divide a program in multiple files. Each file contains related functions. Some functions or macros are required in each program Put them in a file (Library). Include them in program that need them. Nested includes: Included file may have more included files in it.
- 13. 3. Conditional Compilation/ Compiler Control Directives Write single program to run on different environments. #ifdef – if defined #endif – end if #else – else #ifndef – in not defined #if – if #elif – else if
- 14. Where Conditional Compilation is used? To avoide multiple declaration error Instead of comments(Nested comments not allowed) Run the same code on different environment.
- 15. What is a Header File? A header file is a file with extension .h which contains C function declarations and macro definitions to be shared between several source files. There are two types of header files: 1. The files that the programmer writes and 2. The files that comes with your compiler.
- 16. Why Header Files are used? Including a header file is equal to copying the content of the header file but we do not do it because it will be error-prone and it is not a good idea to copy the content of a header file in the source files, especially if we have multiple source files in a program.
- 17. The include syntax Both the user and the system header files are included using the preprocessing directive #include. It has the following two forms − #include <file> #include “file” Example: #include<stdio.h> #include<string.h>